The present invention relates to electric motors and more particularly to motors of the type comprising a stator and a movable member with a multipolar permanent magnet arrangement.
The movable member of such a motor usually has two parallel substantially planar surfaces on which appear the magnetic poles of the permanent magnet arrangement, the thickness of which is substantially smaller than the dimensions of the movable member in the other directions. The magnetization is off a direction perpendicular to such parallel surfaces and perpendicular to the direction of movement of the member, each of the substantially planar surfaces exhibiting at least one series of magnetic poles of the same polarity. The stator of the motor comprises one or more magnetic circuits coupled with at least one electric energizing coil. The magnetic circuit or circuits comprise at least two polar parts facing each other and forming at least part of an air-gap in which the magnetic poles of the movable member are placed.
In known motors of this general type, the air-gap has been dimensioned on the basis of the following consideration. The Ampere-turns ni of the energizing coil are related to the magnetic field H created in the air-gap and to the air-gap width E by the relationship ni=H.multidot.E. The value of ni is limited in particular by the necessary heat dissipation in the stator structure.
Accordingly, in order to increase the torque C delivered by the motor for a given value of ni it is necessary to increase the factor .gamma. appearing in the relationship C=.gamma..multidot.ni which represents the torque per Ampere-turn.
The obvious way to increase the torque per Ampere-turn was to increase the magnetic energy contained in the system and consequently the volume of the permanent magnet. The dimensions of the permanent magnet in the above-mentioned parallel surfaces are limited and determined by the size of the motor and the nubmer of poles required in the multipolar magnet arrangement. The remaining dimension is the one in the direction of the width of the air-gap. Thus, it seems appropriate to choose a relatively large value for the air-gap width E which allows the use of a corresponding relatively thick permanent magnet. This tendency was also supported by the fact that thicker magnets of rare earth materials are less fragile and by the generally good performances, particularly dynamic performances, of the motors of this type as compared to those of other motors of the same power or volume.